Human ESCs were successfully grown in the laboratory for the first time in 1998 (Thompson et al., 1998). Under appropriate culture conditions, ESCs have demonstrated a remarkable ability to self-renew continuously, that is, to produce more cells like themselves that are multipotent. As indicated at the workshop by Thomas Okarma and Ron McKay, ESC lines established from single cells have been demonstrated to proliferate through 300-400 population-doubling cycles. Human ESCs that have been propagated for more than 2 years also demonstrate a stable and normal complement of chromosomes, in contrast to the unstable and abnormal complement of embryonic cancer cell lines used in the past to study early stages of embryonic development. Careful monitoring of the aging ESC lines will be needed to evaluate the significance of genetic changes that are expected to occur over time.

Because human ESCs have only recently become available for research, most of what is known about ESCs comes from studies in the mouse, which, as noted in Chapter 2, cannot be presumed to provide definitive evidence of the capabilities of human cells.

Nevertheless, ESCs derived from mouse blastocysts have been studied for 2 decades and provide a critical baseline of knowledge about the biology and cultivation of these cells (Torres, 1998; Wobus and Boheler, 1999). The factors that permit the mouse ESC to continue replicating in the laboratory without differentiation and methods to trigger differentiation into different cell types that exhibit normal function have been actively explored. Among the types of cells derived from cultured mouse ESCs are fat cells, various brain and nervous system cells, insulin-producing cells of the pancreas, bone cells, hematopoietic cells, yolk sac, endothelial cells, primitive endodermal cells, and smooth and striated muscle cells, including cardiomyocytes—heart muscle cells (Odorico et al., 2001).

Experience with mouse ESCs has provided clues to methods for culturing human ESCs and leading them to differentiate. Mouse ESCs

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